Analog-to-digital converters (ADC) are used to convert analog signals into a digital representation of the same signal. ADCs are used in a wide variety of applications, ranging from medical and entertainment to communications (both voice and data). Analog to digital (A/D) converters are electrical circuit devices that convert continuous signals, such as voltages or currents, from the analog domain to the digital domain, in which the signals are represented by numbers. A variety of A/D converter types exist, including flash A/Ds, sub-ranging A/Ds, successive approximation A/Ds, and integrating A/Ds. Another type is known as a sigma delta or delta sigma (ΔΣ) A/D converter, which includes a delta sigma modulator that samples an analog signal at a very high sampling rate (oversampling) in order to perform a noise shaping function. The oversampling is commonly performed at a multiple of the Nyquist rate (FN) for a given input signal frequency and typically the sampling frequency FS is 10 to 1000 times FN. In this manner, quantization noise power is spread over a bandwidth equal to the sampling frequency, thereby reducing the noise density in the band of interest. Digital filtering is then employed on the digital output to achieve a high resolution. Decimation may then be employed to reduce the effective sampling rate back to the “Nyquist” rate. Delta sigma data converters also typically include a loop filter in the forward signal path that operates to push some of the quantization noise into the higher frequency spectrum beyond the band of interest.
Sigma-delta ADCs are commonly used in applications where high resolution with low to moderate conversion rates are required. An advantage of sigma-delta ADCs is that the sigma-delta ADCs normally make use of single or low multi-bit (two, three, or four bit) quantizers, making the precision requirements of the sigma-delta ADC much lower than other types of ADCs that may use quantizers with a large number of bits (eight or greater). Operating at a frequency greater than the required frequency is commonly referred to as oversampling and an ADC that is operating at a frequency that is K times greater than the required frequency is referred to as a K-times oversampling ADC.
Mobile audio devices are a ubiquitous fixture of modern society. Cellular telephones, personal music players, portable gaming systems, etc. are constant companions for many people. Cell phones continue to increase in computer processing capability and sophistication. The basic radio receiver within the cell phone may make use of a sigma-delta ADC for signal reception and demodulation. The increased memory capacity and computing resources on a cell phone support the installation of various applications, often referred to as “apps” that allow a diverse range of functions to be performed by the cell phone when not being used for conversation. Analog to digital conversion is required by several apps that run on a mobile device and may be performed by a sigma-delta ADC.
US Patent publication 2005/0116850 describes a fourth order delta sigma analog-to-digital converter that has a passive delta sigma modulator including a passive filter, a quantizer, and a digital-to-analog converter in a first feedback loop, and an active filter having a large gain factor in a second feedback loop around the passive delta-sigma modulator.